1. Field of the Invention
The present invention relates to an organic electroluminescent element (also referred to below as an “organic EL element”) which can be employed in flat panel displays and as a light source for illumination. In particular, the object of the invention is to provide a low power consumption transparent organic EL element and a top-emitting organic EL element.
2. Background of the Related Art
Because organic EL elements can be driven at a low voltage and a high current density, they are able to achieve a high luminance and emission efficiency. In recent years, organic EL elements have been put to practical use in flat panel displays such as liquid-crystal displays, and also show promise as a light source for illumination.
An organic EL element has an anode, a cathode, and an organic EL layer sandwiched between the anode and the cathode. Luminescence by an organic EL element is achieved by the emission of light that occurs with relaxation of the excitation energy of excitons generated by the recombination of holes injected into the highest occupied molecular orbital (HOMO) of the emissive layer material within the organic EL layer with electrons injected into the lowest unoccupied molecular orbital (LUMO). The HOMO level of the emissive layer material is generally measured as the ionization potential, and the LUMO level is generally measured as the electron affinity. Generally, to efficiently carry out hole and electron injection into the emissive layer, the organic EL layer has a stacked structure which, in addition to an emissive layer, includes any or all of the following: a hole injection layer, a hole transport layer, an electron transport layer and an electron injection layer.
In an organic EL element, EL light from the emissive layer is emitted from either the anode or the cathode, or from both sides. It is desired that the electrode on the light-emitting side have a high transmittance to EL light from the emissive layer. Transparent conductive oxide (TCO) materials (e.g., indium-tin oxide (ITO), indium-zinc oxide (IZO), indium-tungsten oxide (IWO)) are generally used as such electrode materials. Because TCO materials have a relatively large work function of about 5 eV, an electrode formed of a TCO material may be used as the electrode for injecting holes to the organic EL layer (i.e., as the anode).
Organic EL elements of a type which output light from the supporting substrate side (bottom-emitting (Btm-Em) organic EL elements) have hitherto been common. Such organic EL elements are obtained by forming, on a transparent supporting substrate, an anode composed of a TCO material as the bottom electrode; forming on the anode an organic EL layer having, in order, a hole injection and transport layer, an emissive layer, and an electron injection and transport layer; and forming as the top electrode on the organic EL layer a cathode composed of a film of metal such as aluminum.
Recently, in applications as flat panel displays, active matrix (AM) drive organic EL displays wherein a switching element composed of an amorphous silicon or polysilicon thin-film transistor (TFT) is provided at each pixel and an organic EL element is formed thereon have become predominant, the reason being that displays having a high luminance and a low power consumption can thereby be achieved. At this time, to prevent a decrease in the aperture ratio (light-emitting surface area) of the pixels due to the opacity of the switching elements, it is desirable to employ organic EL elements of a type which has a reflective bottom electrode and a transparent top electrode and emits light from the film formation side (top-emitting (Top-Em) organic EL elements).
With regard to organic EL elements having a transparent top electrode and a reflective bottom electrode, Nature, Vol. 380, (1996), p. 29, describes an organic EL element having a structure that includes a reflective bottom electrode as the anode; an organic EL layer composed of a hole injection/transport layer, an emissive layer, and an electron injection/transport layer which are formed in this order; and a transparent top electrode as the cathode. Applied Physics Letters, Vol. 70, No. 22 (1997), p. 2954, describes an organic EL element having a structure that includes a reflective bottom electrode as the cathode; an organic EL layer composed of an electron injection/transport layer, an emissive layer, and a hole injection/transport layer which are formed in this order; and a transparent top electrode as the anode. Particularly in cases where polysilicon-TFTs are used as the switching elements, it is important for a transparent top electrode to serve as the cathode. This is because, from the standpoint of the switching circuit construction, the bottom electrode is generally used as the anode.
The transparent top electrode is sometimes formed using a metal thin-film of Mg—Ag alloy or the like. In such a case, to obtain a sufficient damage mitigating effect using a metal thin-film, it is necessary to increase the thickness of the metal thin-film. However, increasing the thickness of the metal thin-film leads to a rise in visible light absorbance, resulting in the absorption of EL light from the emissive layer and lowering the intensity of light emission by the organic EL element. Metal thin-films also exhibit a strong microcavity effect attributable to the high reflectance. Due to the microcavity effect, the thickness of the organic EL layer which determines the distance between the reflective bottom electrode and the metal thin-film significantly alters the viewing angle dependence of the emission color and the viewing angle dependence of the emission intensity. Accordingly, there exists a need for very precise control of the film thickness distribution in the organic EL layer (especially the film thickness distribution within the display region). In light of the above, it is desired that the TCO materials hitherto used in anode formation be used in cathode formation.
However, a problem with the emissive layer material and the electron injection/transport material, which are organic substances, is that they readily oxidize when a TCO material is formed thereon by sputtering or the like, causing a deterioration in function and thus a significant loss in the emission efficiency of the organic EL element. An approach hitherto used to prevent oxidative deterioration of the emissive layer material and the electron injection/transport material has been to provide a damage-mitigating electron injection layer between the electron transport layer and the top electrode made of a TCO material. Nature, Vol. 380, (1996), p. 29, proposes using, as such a damage-mitigating electron injection layer, a Mg—Ag alloy thin-film layer hitherto employed as a cathode material. In addition, Applied Physics Letters, Vol. 72, No. 17 (1998), p. 2138, and Japanese Translation of PCT Application No. 2001-520450, propose using, as such a damage-mitigating electron injection layer, a copper phthalocyanine (CuPC) thin-film, a zinc phthalocyanine (ZnPC) thin film or the like.
Japanese Translation of PCT Application No. 2001-520450, states that it is desirable for an organic semiconductor material which, when combined with a TCO layer, is capable of carrying out efficient electron injection and provide the following characteristics.
(1) Sufficient chemical and structural stability to limit damage due to sputtering at the time of ITO layer formation. Large, planar molecules such as phthalocyanine, naphthalocyanine and perylene are preferred. Derivatives of the above compounds in which conjugation by these molecules has been further extended (e.g., compounds in which a benzene ring, naphthalene ring, anthracene ring, phenanthrene ring, polyacene ring or the like has been additionally fused) may be used. In specific circumstances, a polymer material may be used.
(2) Sufficient electron mobility to function as an electron transport layer. Although a larger carrier mobility is preferred, in general, materials having carrier mobilities of at least 10−6 cm2/V s are thought to be sufficient to function as an electron transport layer. In such cases as well, typical examples include large, planar molecules such as phthalocyanine and specific perylene.
Also, as mentioned in Journal of Applied Physics, Vol. 67, No. 1 (1990), p. 528, and Journal of Applied Physics, Vol. 93, No. 5 (2003), p. 2977, oligothiophene compounds are recognized as p-type organic semiconductors having relatively large field-effect hole mobilities (10−4 to 1 cm2/V s). In addition, Applied Physics Letters, Vol. 89, No. 25 (2006), p. 253506, and Japanese Patent Application Laid-open No. 2008-112904, disclose the use of oligothiophene compounds as hole transport materials in organic EL elements.
A damage-mitigating electron injection layer formed of CuPC or the like is able to alleviate the problem of visible light absorption when a metal thin-film is used. However, Applied Physics Letters, Vol. 72, No. 17 (1998), p. 2138, mentions that, with regard to electron injectability from a cathode made of a TCO material into an electron transport layer, a damage-mitigating electron injection layer formed of CuPC or the like is inferior to a Mg—Ag alloy thin-film. The decrease in electron injectability invites a rise in the organic EL element driving voltage. Accordingly, there exists a desire for a damage-mitigating electron injection layer which, in addition to having a good light transmittance and a good damage-mitigating ability when the top electrode is formed by a sputtering process, also has an excellent electron injectability from a cathode made of a TCO material to the electron transport layer.
It is therefore an object of the present invention to provide a damage-mitigating electron injection layer which excels in all of the following: light transmittance, damage-mitigating properties and electron injectability. A further object of the invention is to provide a Top-Em organic EL elements and transparent organic EL elements which use such a layer and have a high efficiency at a low driving voltage.